An alternative penicillin-binding protein involved in Salmonella relapses following ceftriaxone therapy

BACKGROUND: Salmonella causes intracellular infections in humans. Besides quinolones, third generation cephalosporins are first line drugs used for salmonellosis therapy. An unresolved anomaly of this practice involves high relapse rates associated to quinolone- or cephalosporin-susceptible Salmonella isolates in patients that are discharged clinically following initial recovery. Reduced drug accessibility to intracellular locations has been hypothesized to impair pathogen eradication although supporting evidence is lacking in vivo. Here, we uncover a novel penicillin-binding protein as the first Salmonella factor likely contributing to relapse following beta-lactam, mainly ceftriaxone, therapy. METHODS: We used Salmonella enterica serovar Typhimurium mutants lacking the alternative penicillin-binding proteins PBP2(SAL) or PBP3(SAL). Affinity of PBP2(SAL) and PBP3(SAL) for beta-lactam antibiotics was tested. Relapse after ceftriaxone therapy was analysed in the murine typhoid model. FINDINGS: S. Typhimurium does not express PBP2(SAL) or PBP3(SAL) in the Mueller-Hinton medium used for susceptibility testing. The pathogen produces these PBPs in response to acidic pH and nutrient limitation, conditions found in phagosomes of mammalian cells. PBP3(SAL) has low affinity for beta-lactams, even at acidic pH. In vitro susceptibility to ceftriaxone at low pH is strongly reduced. S. Typhimurium lacking PBP3(SAL) was unable to cause relapse in mice following ceftriaxone therapy. INTERPRETATION: The reduced capacity of ceftriaxone to clear S. Typhimurium in vivo is favoured by a switch in beta-lactam targets. This switch, involving production of the less-susceptible PBP3(SAL), remains invisible for standard procedures used in clinical therapy. We conclude that eradication of salmonellosis will be possible only upon targeting of PBP3(SAL) with novel drugs.